Definitions of unilateral offer
an offer made by one party, often a nation state, that may not require any action on the part of another party. Used in this sense, it is similar to a promise or a pledge.
President Asif Ali Zardari made the quite unexpected unilateral offer to make no first use of nuclear weapons in any conflict.
an offer made to the world as a whole rather than to one specific party without the need for further negotiations, especially an offer which requires the offeree to perform an act. The contract is accepted (and therefore binding) when the offeree performs the act, thus forming a binding contract (eg found a lost item and given it back to its original owner in return for a reward).
The manual will constitute, in effect, a unilateral offer that an employee may accept through continued employment.